An organic LED element is one in which an organic layer is put between electrodes, and a voltage is applied between the electrodes to inject holes and electrons, which are allowed to be recombined in the organic layer, thereby extracting light that a light-emitting molecule emits in the course of transition from an excited state to a ground state, and has been used for display, backlight and lighting applications.
The refractive index of the organic layer is from about 1.8 to about 2.1 at 430 nm. On the other hand, the refractive index, for example, at the time when ITO (indium tin oxide) is used as a translucent electrode layer is generally from about 1.9 to about 2.1, although it varies depending on the ITO film-forming conditions or composition (Sn—In ratio). Like this, the organic layer and the translucent electrode layer are close to each other in refractive index, so that emitted light reaches an interface between the translucent electrode layer and a translucent substrate without totally reflecting between the organic layer and the translucent electrode layer. A glass or resin substrate is usually used as the translucent substrate, and the refractive index thereof is from about 1.5 to about 1.6, which is lower in the refractive index than the organic layer or the translucent electrode layer. Considering Snell's law, light which tries to enter the glass substrate at a shallow angle is reflected by total reflection in an organic layer direction, and reflected again at a reflective electrode to reach the interface of the glass substrate again. At this time, the incident angle to the glass substrate does not vary, so that reflection is repeated in the organic layer and the translucent electrode layer to fail to extract the light from the glass substrate to the outside. According to an approximate estimate, it is known that about 60% of the emitted light can not be extracted by this mode (organic layer-translucent electrode layer propagation mode). The same also occurs at an interface between the substrate and the air, whereby about 20% of the emitted light propagates in the glass and fails to be extracted (substrate propagation mode). Accordingly, the amount of the light which can be extracted to the outside of the organic LED element is less than 20% of the emitted light in the present circumstances.
Patent document 1 proposes a structure in which a light scattering layer as a translucent material layer is provided on one surface of a substrate (paragraphs 0039 to 0040). This proposes a structure in which glass particles are firmly fixed to the surface of the substrate with an acrylic adhesive to perform an aggregation arrangement on the surface of the substrate, thereby providing a light scattering portion between the substrate and an organic EL element.
Further, intending to improve extraction efficiency, patent document 2 discloses “an organic EL element comprising a translucent substrate having provided thereon a scattering layer comprising an additional layer composed of a transparent material in which SiO2 particles, resin particles, a metal powder or metal oxide particles are dispersed, by a resin-based adhesive, spraying, vapor deposition, sputtering, dipping, spin coating or the like” (paragraph 0057).
Patent document 3 discloses a light-emitting device in which a diffusing layer obtained by dispersing at least two kinds of fine particles one digit or more different in average particle size in a resin is provided adjacent to a translucent electrode, thereby efficiently extracting wave-guided light.
Further, patent document 4 proposes a technique of preventing total reflection in the inside of a display formed by using a light-emitting device, thereby intending to increase luminance. In patent document 4, it is described that “a high diffusion material may be coated on a layer of a light-emitting device such as a substrate, a transparent electrode, an optical film or another component (patent document 4, paragraph 0027), Furthermore, it is described that “for example, particles may be arranged in glass frit, suitably coated, flattened and fired to form a glass substrate or a layer on a glass substrate, which acts as a high diffusion TIR frustrator” (patent document 4, paragraph 0027).
Moreover, also in patent document 5, paragraph 0026, there are descriptions similar to those of patent document 4.
Patent Document 1 Japanese Patent No. 2931211
Patent Document 2 JP-A-2005-63704
Patent Document 3 JP-A-2005-190931
Patent Document 4 JP-T-2004-513483
Patent Document 5 JP-T-2004-513484